The
phototoxicity of three structurally related
photosensitizers (PS),
deuteroporphyrin IX (DP) and monobromo (Br-DP) and dibromo (Br2-DP) derivatives, was studied in murine
L1210 leukemia cells. These compounds were chosen on the basis of heavy-atom-induced differences in triplet yield, phi T, and lifetime, tau T, and used as tools to test a model for
phototoxicity based on photophysical parameters. All three
porphyrins were found to localize preferentially in the plasma membrane of L1210 cells by confocal fluorescence microscopy. A poor correlation was observed between the measured photodynamic efficacies of these PS and a model using photophysical parameters determined by
laser flash photolysis in homogeneous
solution. However, an excellent correlation was obtained when the same parameters measured directly in the cells were used. The
biological microenvironment of the
porphyrins in cells induces significant changes in the photophysics of the PS. Reduction in fluorescence yield, phi F, and phi T observed for Br2-DP in cell
suspensions arises from self association of the molecule due to increased hydrophobicity and high local concentrations. The photophysical model was also tested for its ability to handle variations in the
oxygen dependence of cellular
phototoxicity of these PS. The good correlation achieved between
laser flash photolysis data determined in cells and the measured
phototoxicity under air, 1.5% and 0.5% O2-saturated conditions, proves the intermediacy of
singlet oxygen. This study gives further credence to the direct use of photophysical techniques to elucidate photochemical mechanisms in
biological media while highlighting the potential pitfalls of using
solution data to predict photosensitizing potential.